Conventionally, the negative PS plate has been widely known and includes those using a photosensitive layer containing a diazo resin which is cured on exposure to light, those using a photopolymerizable photosensitive layer and those using a photo-crosslinkable photosensitive layer. The term "photopolymerizable" or "photo-crosslinkable" as used herein means such a property that the organic layer is caused to undergo polymerization or crosslinking due to an active species generated by light. To make a printing plate from such a PS plate, an original of a transparent negative film is placed on the PS plate having the above-described photosensitive layer and the plate is then exposed to an ultraviolet ray. Even when the original comprises letters, a part of the original must be once reproduced on the film before exposing the PS plate to the ray, thus, the operation is cumbersome. Accordingly, attempts have been intensively made to directly prepare a printing plate without using any film original, where a PS plate using a certain high-sensitive, polymerizable layer as the photosensitive layer is scanned with an acutely narrowed laser beam to form a letter original or an image original directly on the plate. A printing plate can be directly made using a photosensitive composition described, for example, in JP-B-61-9621 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-63-178105 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-2-244050, without any aid of a film original.
However, these conventional high-sensitive, photopolymerizable printing plates are not always strong in the adhesion between the photosensitive layer and the support, though it may depend on the conditions on use as the printing plate, and therefore, when a large number of sheets are printed at a high speed, there arise failures such that a solid image falls or a thin line drops out. In order to achieve strong adhesion between the photosensitive layer and the support, the roughened surface on the support surface may be rendered coarser and thereby the image can be surely improved in durability, however, in turn, the non-image area is reduced in hydrophilicity and as a result, scums are generated in printing. Further, when an undercoat layer such as a diazo resin is used to improve the adhesion between the photosensitive layer and the support, the diazo group photolyzes so that the process of the diazo group itself bonding to the support or the process of the diazo group itself bonding to the photosensitive layer difficultly proceeds, and moreover, a relatively large quantity of exposure amount is required, hence, this technique is not suitable for the system of making a printing plate directly by exposure to laser light.
Under these circumstances, attempts have been made to integrate a larger number of polymerizable functional groups into a sol-gel processing solution so that light adhesion can more efficiently proceed (see, for example, JP-A-7-159983), however, if too many polymerizable functional groups are integrated, hydrophilicity of the non-image area is impaired and a problem arises that scums are generated.